In pneumatic braking systems for heavy-duty trucks, air is vented from the pneumatic lines to release the service brakes. The lag time between the actual release of a brake pedal and the actual release of the vehicle brakes is referred to as brake release timing. Users desire brake release timing to be as small as possible. To enhance brake release timing, quick release valves are commonly employed. Quick release valves are typically installed in the air brake system at a point between the supply air volume and the delivery air volume. Upon release of the brake pedal, air that was delivered to the brakes is rapidly vented from the quick release valve instead of flowing all the way back to the air supply.
A known type of quick release valve is illustrated in FIG. 1. The valve 10 includes a housing 12 having a supply port 14, an exhaust port 16, and one or more delivery ports 18. A flexible, disk-shaped diaphragm 20 resides in the housing 12 for sealing the supply port 14 or the exhaust port 16 when desired.
In operation, when the driver depresses the brake pedal, air flows into the supply port 14 causing the diaphragm 20 to seal against an exhaust seat 22. At the same time, the air forces the outer edge of the diaphragm 20 downward, resulting in air flowing from the supply port 14 to the delivery ports 18. When the driver releases the brake pedal, the air pressure at the supply port 14 is reduced. The air that had been delivered to the brakes flows back toward the supply port 14. The differential pressure across the diaphragm 20 (i.e. higher pressure in the delivery port than the pressure in the supply port) moves the diaphragm 20 upward, away from the exhaust seat 22, and into engagement with an inlet seat 24. As a result, air from the delivery volume vents through the exhaust port 16.
In some known quick release valves, a differential pressure occurs across the diaphragm when air flows from the supply port to the delivery ports. This is not desirable because it may result in an unbalanced pressure between the wheels of the vehicle. Furthermore, in some designs, the diaphragm will not establish a seal with the inlet port during brake release as rapidly as desired, especially during low-pressure applications, such as for example 30 psi or less. As a result, some delivered air may flow back into the supply port, thus degrading brake release timing.